1. Field of the Invention
The present invention relates to an in-wheel motor system for a steering wheel for use in a vehicle having direct drive wheels as steering wheels.
2. Description of the Prior Art
In recent years, in a vehicle driven by a motor such as an electric car, an in-wheel motor system which incorporates a motor composed of a knuckle as a frame dress-up part and a drive motor in wheels has been employed due to its high space efficiency and drive force transmission efficiency (for example, U.S. Pat. No. 2,676,025, JP-A 9-506236 and JP-A 10-305735) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”).
However, since the motor is fixed to a knuckle which is a frame dress-up part of the vehicle in the above in-wheel motor system of the prior art, when an in-wheel motor is used in the steering wheel, the motor turns in a steering direction together with the wheel at the time of steering. That is, as the inertia moment on the steering axis of the steering wheel provided with the in-wheel motor increases due to the mass of the motor, not only does the steering torque becomes large but also the resonance in the steering direction easily occurs.
In a vehicle having a suspension mechanism such as a spring, it is known that as the mass of unsprung parts such as a wheel, knuckle and suspension arm so called “unsprung mass” increases, variations in the ground contact force of a tire become larger and the road holding properties become worse when the vehicle runs on a rough road. In the in-wheel motor of the prior art, as the motor is fixed to the knuckle as described above, the above unsprung mass increases by the mass of the motor with the result that variations in the ground contact force of the tire become larger and the road holding properties become worse.
It is an object of the present invention which has been made in view of the above problems of the prior art to provide an in-wheel motor system for a steering wheel which can reduce an increase in the steering torque of a steering wheel provided with an in-wheel motor.